1. Technical Field
The disclosed embodiments relate to receiver DC offset calibration, and more particularly, to DC offset calibration in multiple-input multiple-output (MIMO) applications using a DC offset calibration table.
2. Background Information
FIG. 1 (Prior Art) is a simplified block diagram of a radio frequency (RF) transceiver integrated circuit 11. RF transceiver integrated circuit 11 comprises a transmitter as well as a plurality of receivers. Each receiver includes a “receiver chain” (i.e., RX1, RX2, and RX3) as well as a local oscillator (LO) that supplies LO signals onto the receiver chain. For example, in receiving mode, a high frequency RF signal 14 is received on antenna 13. Information from signal 14 passes through matching network 15 and through a receiver chain 12 (RX1). Signal 14 is amplified by Low Noise Amplifier (LNA) 16 and is down-converted in frequency by mixer 17. The resulting down-converted signal is amplified by transimpedance amplifier (TIA) 18, filtered by baseband filter (BBF) 19, and then passed to a digital baseband integrated circuit (not shown) for digital processing.
In a zero- or low-intermediate frequency (IF) system, any DC offset of a receiver chain is calibrated to ensure proper signal reception and processing. The DC offset is generally caused by the mismatch of different circuits of the receiver (i.e., TIA, BBF etc.), and also by LO leakages to the receiver front end. In the example of FIG. 1, LO signals (i.e., LO-I and LO-Q) are provided by a phase-lock-loop (PLL) block and a plurality of divide-by-2 circuits to the mixer in each receiver chain. The LO signals, however, leak onto the input of the LNA of each receiver. The LO leakages are then mixed with the original LO signals at the mixer and fall into a DC offset voltage. In multiple-input multiple-output (MIMO) or multiple-input single-output (MISO) systems, LO leakages come from multiple divide-by-2 circuits or the PLL block for multiple receiver chains. Efficient and accurate DC offset calibration remains a challenge in MIMO/MISO systems.